1. Field of the Invention
The present invention relates to switching power conversions, and more particularly to switching power conversions capable of adjusting switch-off timing of the secondary side current path of a main transformer.
2. Description of the Related Art
In supplying the power for electronic equipments, switching power converters are widely adopted due to the advantages of high conversion efficiency and small component size they possess.
Taking the fly-back AC-to-DC power adapter as an example, FIG. 1a shows an illustrating diagram of a fly-back AC-to-DC power adapter in the charging period of a transformer and FIG. 1b shows an illustrating diagram of a fly-back AC-to-DC power adapter in the discharging period of a transformer. As shown in FIG. 1a and FIG. 1b, the shown architecture includes an NMOS transistor 101, a main transformer 102, a diode 103 and a capacitor 104.
In the architecture, the NMOS transistor 101 is used to control the power transformation through the main transformer 102 in response to a PWM signal VG1.
The main transformer 102 is used to transfer the input DC power VIN to a DC output voltage VCC.
The diode 103 is coupled with the secondary side of the main transformer 102 for cutting off the current path at the secondary side when the NMOS transistor 101 is on and releasing the magnetic flux to the capacitor 104 when the NMOS transistor 101 is off. When the NMOS transistor 101 is on, the cathode voltage of the diode 103 is VIN/N+VCC, greater than the anode voltage GND of the diode 103, causing the diode 103 reverse biased, so the current path at the secondary side is cut off; when the NMOS transistor 101 is off, the voltage across the secondary side of the main transformer 102 is reversed in polarity, causing the cathode voltage of the diode 103 smaller than the anode voltage of the diode 103, so the current path at the secondary side is turned on.
The capacitor 104 is used for carrying the DC output voltage VCC.
Through a periodic on-and-off switching of the NMOS transistor 101, which is driven by the PWM signal VG1 generated from a PWM controller (not shown in the figures), the input power is transformed through the main transformer 102 to the output.
However, when the magnetic flux is released through the diode 103, the conduction voltage 0.7V of the diode 103 will consume quite an amount of energy and degrade the conversion efficiency, especially when the DC output voltage VCC is rated at a low voltage.
One solution that conventional power converters utilize to solve this problem is to replace the diode 103 with a switch circuit having a lower conduction voltage to improve the conversion efficiency.
Please refer to FIG. 2, which shows a prior art circuit diagram for switching the secondary side of a transformer of a fly-back AC-to-DC power adapter. As shown in FIG. 2, the prior art circuit includes a diode 201, a comparator 202 and an NMOS transistor 203.
The diode 201 is used to handle instances where the switching speed of the comparator 202 and the NMOS transistor 203 is slower than the switching speed of the input signals.
The comparator 202 and the NMOS transistor 203 are used to emulate the function of a diode. The comparator 202 controls the conduction of the NMOS transistor 203 in response to the anode voltage and the cathode voltage of the diode 201. When the anode voltage exceeds the cathode voltage by a threshold voltage, the comparator 202 will turn on the NMOS transistor 203 and the resulting conduction voltage will be much smaller than that of the diode 201, otherwise the comparator 202 will turn off the NMOS transistor 203. The relation between the conduction current I and the conduction voltage VF of the circuit in FIG. 2 is depicted in FIG. 3. As shown in FIG. 3, when VF exceeds 0.25 mv, the conduction current I will increase with a slope equal to 1/RDSON; when VF falls below 0.25 mv, the conduction current I will reduce to zero. Although this prior art circuit has the advantage of reducing the conduction voltage, there are still two major cons. First, the threshold voltage of 0.25 mv requires a comparator with superior resolution, which is not easy to be implemented. Second, it is difficult to determine the value of the threshold voltage in CCM (Continuous Current Mode). Please refer to FIG. 4, when the power adapter operates in CCM, if the loading condition changes from low load to high load such that the valley current of the high load exceeds a threshold current corresponding to the threshold voltage set previously (threshold voltage=threshold current×RDSON), then the switch circuit will never be triggered to cut off the secondary side current path and this may lead to system disaster.
To solve the secondary side current path cut-off problem in CCM, the U.S. Pat. No. 6,771,059B1 proposes to measure the cycle period of the cathode voltage of the secondary side diode by detecting a high voltage VIN/N+VCC (shown in FIG. 1a), an instance corresponding to the conduction of the primary side, and then cut off the secondary side at the time according to the cycle period. However, this scheme can not be applied to the DCM (Discrete Current Mode) case, because, in DCM, the energy in the main transformer has already dried up before the conduction of the primary side, as a result, there will be a reverse current from an output capacitor in the secondary side current path, and the reverse current will interfere with the charging of the main transformer at the conduction of the primary side.
Therefore, there is a need to provide a solution capable of switching off the secondary side current path appropriately both in CCM and DCM.
Seeing this bottleneck, the present invention proposes a novel topology for generating an off-predicting signal capable of appropriately switching off the secondary side current path both in CCM and DCM to prevent reverse current in the secondary side current path.